We conducted a new thermodymics analysis of diffusive transport in geologic systems with multiple partially-soluble fluids, focusing on the important cases of CO2 and H2. We show that in the presence of a geothermal gradient, entropic contributions can reverse the direction of diffusion! This was completely counter-intuitive (to me) and a fascinating result.We predict that for essentially all realistic geologic carbon storage scenarios, diffusion will drive CO2 downwards. For underground hydrogen storage, the picture is more complicated; H2 will likely diffuse upwards but might reverse direction in deep or high-temperature gradient systems.
For details on our analysis, check out the publication in Physical Review E.